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Members of the Branchiopoda are unified by the presence of gills on many of the animals' appendages, including some of the mouthparts. This is also responsible for the name of the group[1] (from the Greek: branchia, gills, akin to bronchos, windpipe; Greek: pous, foot).[2] They generally possess compound eyes and a carapace, which may be a shell of two valves enclosing the trunk (as in most Cladocera), broad and shallow (as in the Notostraca), or entirely absent (as in the Anostraca).[3] In the groups where the carapace prevents the use of the trunk limbs for swimming (Cladocera, clam shrimp and the extinct Lipostraca), the antennae are used for locomotion, as they are in the nauplius.[3] Male fairy shrimp have an enlarged pair of antennae with which they grasp the female during mating, while the bottom-feeding Notostraca, the antennae are reduced to vestiges.[3] The trunk limbs are beaten in a metachronal rhythm, causing a flow of water along the midline of the animal, from which it derives oxygen, food and, in the case of the Anostraca and Notostraca, movement.[3]

In early taxonomic treatments, the current members of the Branchiopoda were all placed in a single genus, Monoculus. The taxon Branchiopoda was erected by Pierre André Latreille in 1817, initially at the rank of order.[5]

The fairy shrimp of the order Anostraca are usually 6–25 mm (0.24–0.98 in) long (exceptionally up to 170 mm or 6.7 in).[6] Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia (swimming legs), and the body lacks a carapace.[7] They live in vernal pools and hypersaline lakes across the world, including pools in deserts, in ice-covered mountain lakes and in Antarctica. They swim "upside-down" and feed by filtering organic particles from the water or by scraping algae from surfaces.[6] They are an important food for many birds and fish, and are cultured and harvested for use as fish food.[8] There are 300 species spread across 8 families.[9]

The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp.[15] The two genera, Triops and Lepidurus, are considered living fossils, having not changed significantly in outward form since the Triassic.[9] They have a broad, flat carapace, which conceals the head and bears a single pair of compound eyes.[6] The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs.[6] The telson is flanked by a pair of long, thin caudal rami.[3]Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction.[9] Notostracans are the largest branchiopodans and are omnivores living on the bottom of temporary pools and shallow lakes.[6]

Clam shrimp are bivalved animals which have lived since at least the Devonian. The three groups are not believed to form a clade. They have 10–32 trunk segments, decreasing in size from front to back, and each bears a pair of legs which also carry gills. A strong muscle can close the two halves of the shell together.